The invention relates to an analog-to-digital converter for converting an analog input signal into an offset-free digital output signal.
Digital signal processing systems are in widespread use. Since digital signal processing systems have numerous technical and economical advantages over analog circuits, digital signal processing systems are replacing the conventional analog circuits to an increasing extent.
The transmission of signals is analog in many cases on account of an only limited bandwidth available in the transmission medium, with the result that an analog-to-digital converter is connected upstream of the digital signal processing systems.
It is advantageous for the further data processing of the converted digital signal if the converted digital signal is offset-free, i.e. has no DC component. A DC offset compensation on the one hand has the effect that the digital output data converted by the analog-to-digital converter have a zero mean value, and on the other hand increases the dynamic range of the analog-to-digital converter.
U.S. Pat. No. 5,523,756 describes an analog-to-digital converter having feedback for offset reduction. The analog-to-digital converter described therein detects the presence of an analog input signal with a mean value. The analog input signal is converted into a digital output signal by an analog-to-digital converter, the less significant bits of the digital output signal being compared with a threshold value in order to generate an indicator signal which specifies whether or not an analog input signal is present. DC-component offsets in the digital output signal are compensated by the generation of a DC compensation signal based on the most significant bit of the digital signal with respect to a reference voltage. The DC compensation signal is added to the analog input signal prior to conversion into the digital output signal. The DC compensation signal or actuating signal is generated by means of analog filtering of the digital output signal.
However, analog filtering of the digital output signal for the purpose of generating an actuating signal for DC offset compensation of the analog-to-digital converter has a number of disadvantages.
A circuitry realization of analog filtering is relatively area-intensive in the integration of the analog-to-digital converter, i.e. a relatively large chip area is required for it.
Furthermore, external circuit components, such as coupling capacitors, for example, are necessary.
A further disadvantage is the relatively inflexible circuitry implementation of the analog filtering.
The object of the present invention, therefore, is to provide an analog-to-digital converter with offset compensation which can be realized in a simple manner in terms of circuitry and can be implemented flexibly.
This object is achieved according to the invention by means of an analog-to-digital converter having the features specified in patent claim 1.